1. Field of the Invention
The present invention relates to an image reading apparatus and more particularly it relates to an image reading apparatus for reading a transparent original rested on an original glass plate.
2. Related Background Art
In the past, in a case that a transparent original such as a photographic film is read by an image reading apparatus of flat head type having an original glass plate, there has mainly be used a system in which the transparent original is firstly installed in an transparent original guide unit and then the transparent original guide unit is rested on the original glass plate and then the transparent original is illuminated from above by means of a surface light source unit. The image reading apparatus according to this system is generally designed so that a position completely focused becomes a position having a distance of 0 mm from an upper surface of the original glass plate, i.e. a position contacted with the original glass plate. The reason is that it is considered or supposed that a reflection type original is mainly used. In a case that the transparent original is used, generally, the transparent original is installed at a distance of about 0.5 mm above the original glass plate by means of a transparent original guide.
In this system, in a case that the image is picked up on CCD by using a lens of minification optical system, since a depth of field is deep, even if the distance between the original glass plate and the transparent original is about 0.5 mm, there is no practical problem. Further, if the transparent original is curled more or less, there arises no problem.
However, in recent years, there has been proposed a reading unit according to an equal magnification optical system using a rod lens array so as to attain miniaturization. Since the depth of field of the rod lens array is shallow, if the distance between the original glass plate and the transparent original is about 0.5 mm or if the transparent original is curled, there arises a problem regarding out of focus.